A. Field of Invention
This invention relates to thermoplastic splint and cast forming material in the form of a bandage, web, film, tape or sheet material. Such splints and casts are useful in the treatment of the human and animal body for the maintenance of immobilization, fixation and bracing following reduction of fractures and dislocations and the maintenance of approximation of bone fragments following reduction of fractures. The invention relates to the use of crystalline polyurethane polymers which contain a filler and which may be softened by heat and shaped to conform to the body and which recrystallize upon cooling to form a rigid, self-supporting cast or splint.
B. Description of the Prior Art
Heretofore, cast materials such as plaster of Paris have been extensively used but casts made from such materials are heavy, bulky, sensitive to water and have poor X-ray penetrability. Aluminum splints, lined with soft polyether foam, have also been used but are difficult to properly shape and to fasten to the portion of the body intended to be immobilized.
The use of specific thermoplastic materials in casts and splints is known. U.S. Pat. No. 2,385,879 discloses the use of a copolymer of a vinyl ester of an aliphatic acid and a vinyl halide. U.S. Pat. No. 3,420,231 discloses a fibrous substrate coated with a mixture of trans-1,4-choroprene and an inversely soluble resin such a methyl cellulose. U.S. Pat. No. 3,442,265 discloses the use of polymethyl methacrylate and U.S. Pat. No. 3,809,600 discloses transpolyisoprene. Foamed thermoplastic materials such as polyethylene, U.S. Pat. No. 2,947,307, and foamed polyurethane formed in situ, U.S. Pat. No. 3,301,252, have also been disclosed.
These prior materials and methods for forming splints, braces, supports or casts have met with little practical success because of one or more disadvantages attending their use. In some cases, they are difficult to apply or mold and involve complicated heating and water treatments or other manipulating steps. In other cases, separation of components, such as plasticizers, from the splint, brace, support or cast containing same can cause discomfort and in some instances, extreme irritation to the skin of the patient. In still other cases, the splint, brace, support or cast is water sensitive, lacks sufficient strength or rigidity, is difficult to reliably fasten to the body portion being corrected and/or is difficult to remove when no longer needed. Crystalline polymers, which melt at temperatures of from about 35.degree. to 100.degree. C. can be used as splints, or casts provided such polymer can be molded at a temperature which will not harm the skin. These materials rely on the recrystallization to provide strength to the splint or cast; the amorphous polymer being pliable. Many of these materials can be "worked" at temperatures below the recrystallization temperature for a time so that the splint or cast may be formed at a comfortable temperature and only time is required to obtain strength through recrystallization. However, many of these materials recrystallize at too slow a rate taking for up to thirty minutes while waiting for sufficient recrystallization of the polymer to occur. Of course, the polymer may be artificially cooled, for example, by placing in cold water. This additional step is inconvenient for both the patient and the orthopedist. U.S. Pat. No. 4,105,025 discloses the use of a crystallizable polyurethane polymer applied to a fibrous substrate. The use of this polymer avoids some of the problems that were encountered with the thermoplastic polymers previously mentioned. However, the splint or cast materials, as disclosed in U.S. Pat. No. 4,105,025, do not have adequate hardness or modulus properties. When such casts or splints are applied to a body, the heat of the body softens the materials to the point where they will no longer provide adequate support. In addition, the recrystallization or set time, that is, the time required for the unfilled polymer cast or splint to reharden after it has been softened by heat, is too long to be acceptable. The set time should be ten minutes or less.
It would be expected that the addition of inert fillers could be employed to improve the hardness and modulus properties of the polyurethane polymer casts or splints. It has previously been determined that the simple addition of a filler, e.g., wollastonite or a filler and coupling agent to the polymer will not overcome these deficiencies. If sufficient filler is added to the polymer to give adequate hardness, the cast material becomes excessively brittle on recrystallization. The excessively brittle casts tend to crack and break and become useless as a support for a limb. The excessive brittleness is particularly detrimental when the cast material is used in a body cast or a scoliosis jacket. A scoliosis jacket is often removed and replaced on a patient. If the cast material is brittle, the cast will break when it is removed.